This invention relates to distributed feedback devices such as distributed feedback lasers and to improvements thereof which remove mode degeneracy.
The first distributed feedback devices, and particularly distributed feedback lasers, were proposed by one of us, Mr. C. V. Shank and Mr. H. W. Kogelnik, in Pat. No. 3,760,292 issued Sept. 18, 1973. The realization of such a laser, as described in that patent, utilizes the Bragg reflection for a spatially periodic variation in index of refraction, gain coefficient or waveguide propagation constant, the most typical example of the latter being a variation in guide thickness. Numerous other distributed feedback lasers have been described in the literature, for example, by H. W. Yen et al in the article entitled "Optically Pumped GaAs Waveguide Lasers With a Fundamental 0.11.mu. Corrugation Feedback," Optics Communications, Volume 9, page 35 (September 1973), by E. J. Staples et al, in their article, 1974 Ultrasonic Symposium Proceedings, page 245, and by J. S. Schoenwald et al, ibid., page 253.
Distributed feedback structures have the advantage of providing better frequency selectivity of the mode of oscillation than resonators formed by partially transmissive reflectors at the ends of the structure. Furthermore, periodic structures with distributed feedback have other applications. Bandpass transmission and reflection filters may be constructed using passive structures that employ Bragg reflections. For example, see the article by R. C. M. Li et al, 1974 Ultrasonic Symposium Proceedings, page 257. The reflection filter formed by such a structure is the equivalent of a mirror reflecting a band of frequencies.
The range of application now foreseen for the distributed feedback principle motivates a study of variations of the distributed feedback design with the objective of obtaining improved performance for many uses. One of the objectives of concern in this case is the mode spectrum of a typical distributed feedback laser, as analyzed by Kogelnik and Shank, Journal of Applied Physics, Volume 43, page 2327 (1972). That mode spectrum exhibits a gap at a center frequency. Modes of equal threshold occur symmetrically on the two sides of this gap. This threshold degeneracy is a disadvantage in practical applications in which single mode operation at a predictable frequency is desired, since one cannot predict which of the symmetrically placed modes will take hold first and control the operation of the device.
It would be desirable to obtain distributed feedback devices not characterized by such mode degeneracy.